Update python-dateutil to 2.8.1

[pyup-bot]

This PR updates python-dateutil from 2.8.0 to 2.8.1.

Changelog

2.8.1

==========================

Data updates
------------

- Updated tzdata version to 2019c.


Bugfixes
--------

- Fixed a race condition in the ``tzoffset`` and ``tzstr`` "strong" caches on
Python 2.7. Reported by kainjow (gh issue 901).
- Parsing errors will now raise ``ParserError``, a subclass of ``ValueError``,
which has a nicer string representation. Patch by gfyoung (gh pr 881).
- ``parser.parse`` will now raise ``TypeError`` when ``tzinfos`` is passed a
type that cannot be interpreted as a time zone. Prior to this change, it
would raise an ``UnboundLocalError`` instead.  Patch by jbrockmendel (gh pr
891).
- Changed error message raised when when passing a ``bytes`` object as the time
zone name to gettz in Python 3.  Reported and fixed by labrys () (gh issue
927, gh pr 935).
- Changed compatibility logic to support a potential Python 4.0 release. Patch
by Hugo van Kemenade (gh pr 950).
- Updated many modules to use ``tz.UTC`` in favor of ``tz.tzutc()`` internally,
to avoid an unnecessary function call. (gh pr 910).
- Fixed issue where ``dateutil.tz`` was using a backported version of
``contextlib.nullcontext`` even in Python 3.7 due to a malformed import
statement. (gh pr 963).


Tests
-----

- Switched from using assertWarns to using pytest.warns in the test suite. (gh
pr 969).
- Fix typo in setup.cfg causing PendingDeprecationWarning to not be explicitly
specified as an error in the warnings filter. (gh pr 966)
- Fixed issue where ``test_tzlocal_offset_equal`` would fail in certain
environments (such as FreeBSD) due to an invalid assumption about what time
zone names are provided. Reported and fixed by Kubilay Kocak (gh issue 918,
pr 928).
- Fixed a minor bug in ``test_isoparser`` related to ``bytes``/``str``
handling. Fixed by fhuang5 (gh issue 776, gh pr 879).
- Explicitly listed all markers used in the pytest configuration. (gh pr 915)
- Extensive improvements to the parser test suite, including the adoption of
``pytest``-style tests and the addition of parametrization of several test
cases. Patches by jbrockmendel (gh prs 735, 890, 892, 894).
- Added tests for tzinfos input types. Patch by jbrockmendel (gh pr 891).
- Fixed failure of test suite when changing the TZ variable is forbidden.
Patch by shadchin (gh pr 893).
- Pinned all test dependencies on Python 3.3. (gh prs 934, 962)


Documentation changes
---------------------

- Fixed many misspellings, typos and styling errors in the comments and
documentation. Patch by Hugo van Kemenade (gh pr 952).


Misc
----

- Added Python 3.8 to the trove classifiers. (gh pr 970)
- Moved as many keys from ``setup.py`` to ``setup.cfg`` as possible.  Fixed by
FakeNameSE, aquinlan82, jachen20, and gurgenz221 (gh issue 871, gh pr
880).
- Reorganized ``parser`` methods by functionality. Patch by jbrockmendel (gh
pr 882).
- Switched ``release.py`` over to using ``pep517.build`` for creating releases,
rather than direct invocations of ``setup.py``. Fixed by smeng10 (gh issue
869, gh pr 875).
- Added a "build" environment into the tox configuration, to handle dependency
management when making releases. Fixed by smeng10 (gh issue 870,r
gh pr 876).
- GH 916, GH 971

Links

• PyPI: https://pypi.org/project/python-dateutil
• Changelog: https://pyup.io/changelogs/python-dateutil/
• Docs: https://dateutil.readthedocs.io